Caprolactam and lauryllactam are respectively the pre-cursors of polyamides 6 and 12. Industrially, the conventional process for synthesizing lactams from cyclic ketoximes is based on a reaction in an acidic medium, for example sulfuric acid. Large amounts of acid are necessary and the recovery of the lactam necessitates the neutralization of the medium by a base. Consequently, in addition to the disadvantages related to the use of acid, the latter has to be neutralized after the reaction. It therefore cannot be recycled and results in a significant by-production of undesirable discharges (by-production of 3 to 5 tonnes of ammonium sulfate per tonne of lactam when sulfuric acid is used and when the medium is neutralized with ammonia). Various processes, in particular using heterogeneous catalysis, have been envisaged in order to avoid this by-production of products which cannot be recovered in value.
In a recent paper published in Studies in Surface Sciences and Catalysis (2001, 135, 3719-3726), Maria Climent et al. describe the synthesis of delaminated zeolites and their applications in organic synthesis. They describe in particular the use of these solids for the Beckmann rearrangement in a batchwise liquid-phase process. Dealuminated β zeolite is mentioned among the solids given as examples; the conversion and the selectivity obtained with this zeolite for the rearrangement of cyclododecanone oxime to L12 are 16 and 98% respectively. The selectivity is thus high; in contrast, the conversion remains low with this type of catalyst. Furthermore, there is no information on the gas-phase Beckmann rearrangement.
The Beckmann rearrangement has also been operated by heterogeneous catalysis in the gas phase, in particular for the synthesis of caprolactam. Specifically, processes have been developed although these are compounds with high boiling points (206-210° C. for cyclohexanone oxime and 139° C./12 mmHg for caprolactam). Apart from looking for the good catalyst which will result in a high catalytic activity, the major difficulty is the stability of the catalyst, which rapidly deactivates as a result of adsorption/decomposition of the organic compounds. In the case of the rearrangement of cyclododecanone oxime, the organic compounds are even heavier than those of the series comprising six carbon atoms. This is why, even if a few short-duration tests are described in the literature, an industrial process appears to be difficult to envisage due to the difficulty in maintaining the catalytic activity.
U.S. Pat. No. 3,586,668 discloses the synthesis of lactam by gas-phase rearrangement over catalytic systems consisting of a mixture of boron trioxide or of boric acid and of carbon which is highly dispersed (particle size of less than 0.1 mm) and in the presence of water. This patent discloses and exemplifies in particular the synthesis of caprolactam; however, the final example (Example 8) describes the synthesis of lauryllactam over a catalyst having a particle size of between 1 and 2 mm. The test described in this example is of short duration (5 hours 15 minutes) and, although the content of L12 in the final mixture is mentioned (86%), it is not possible to deduce therefrom the reaction yield since the weight of product obtained is not indicated. Furthermore, no information on the stability and the lifetime of the catalyst is given.
The problems encountered during a gas-phase Beckmann rearrangement over a heterogeneous catalyst are mentioned in Patent Application JP 48012754 A (registration No. 44-76676 of 7 Oct. 1969 on behalf of Asahi Chemical Industry). Inter alia, the inventors bring up the problem of very rapid deactivation of the catalyst related to the thermal decomposition and the polymerization of the organic compounds on the solid. To overcome these difficulties, in particular for cyclododecanone oxime, the inventors recommend the use of catalysts composed of boric acid deposited on a porous support, such as diatoms. Example 1 of this patent describes the rearrangement of cyclododecanone oxime over a catalyst of this type. Although repeated 30 times, the test is of short duration (5 minutes). It is mentioned that the conversion is stable if the temperature is 300° C.; in contrast, no information is given with regard to the selectivity for or the yield of L12.
The synthesis of caprolactam by gas-phase heterogeneous catalysis over β zeolites or zeolites of MFI type is claimed in U.S. Pat. No. 6,051,706 and U.S. Pat. No. 6,071,844. Out of concern for improving the performances of the catalysts, in particular for reducing their deactivation, on the one hand, the cyclohexanone oxime is dissolved in ethanol and, on the other hand, the catalysts are modified (use of zeolites comprising boron and treatment of the MFIs in order to remove all or part of the central metal atoms). Despite this, the inventors were driven to work on the regeneration of the catalyst in the presence of air or of nitrogen. The synthesis of L12 is not mentioned in these patents since they are restricted to the synthesis of caprolactam.
Patent Application WO 2004/037785 discloses the catalytic rearrangement of cyclododecanone oxime to give lauryllactam (L12) either (i) in the gas phase or (ii) in the liquid phase in the presence of a catalyst which is a monolaminar silicate with an acidic nature. This catalyst is prepared by delamination of the precursor of a zeolite with a laminar structure; this is therefore not a zeolite.
The prior art has described the catalytic rearrangement of cyclododecanone oxime to give lauryllactam (L12) either (i) in the gas phase over catalysts which are not zeolites (FR 1 562 298 and JP 48012754 A) or (ii) in the liquid phase over zeolites (Studies in Surface Sciences and Catalysis (2001, 135, 3719-3726)). A zeolite is defined as a microporous material having a three-dimensional inorganic main structure composed of tetrahedra connected via a common edge (fully linked corner-sharing tetrahedra); see in particular “Nomenclature of structural and compositional characteristic of ordered microporous and mesoporous materials with inorganic hosts. L. B. McCusker, F. Liebau, G. Engelhardt, Pure Appl. Chem., 73, pp. 381-394, 2001”.
It has now been found that it is possible to carry out the catalytic rearrangement of cyclododecanone oxime to give lauryllactam in the gas phase over a zeolite.